Comet ISON arrives at perilous encounter with sun

In this frame grab taken from enhanced video made by NASA's STEREO-A spacecraft, comet ISON, left, approaches the sun on Nov. 25, 2013. Comet Encke is shown just below ISON, The sun is to the right, just outside the frame. ISON, which was discovered a year ago, is making its first spin around the sun and will come the closest to the super-hot solar surface on Thanksgiving Day, Thursday, Nov. 28, 2013, at 1:37 p.m. EST. (AP Photo/NASA)

Comet ISON's day of reckoning has arrived at last.

On Thanksgiving Day the icy comet is due to whip around the sun at more than 200 miles per second, the harrowing culmination of a journey that began about 5{ million years ago in the farthest reaches of the solar system. ISON will experience temperatures of 5,000 degrees Fahrenheit as the sun's powerful gravity threatens to tear it apart.

Many thousands of Earthlings have been tracking the comet's journey for more than a year since it was spotted by Russian astronomers in the neighborhood of Saturn's orbit. So have nearly two dozen spacecraft, including NASA's Hubble and Spitzer telescopes.

For ardent sky watchers, it has been a reality show unlike any other. And now comes the cliffhanger: Will Comet ISON survive?

"We are all standing side-by-side as we witness a complete scientific mystery unfold before us," astrophysicist Karl Battams, NASA's primary Comet ISON blogger, wrote in a post this week. "We have absolutely no idea if ISON will survive past the Sun or not, and how it might look in our December night skies, if it ever gets that far."

Whatever happens, the space rock has been a gift to science. It is already the most-observed comet in NASA's history, and the data scientists have collected will help them study the sun and how our solar system came to be.

Carey Lisse, an astronomer at the Johns Hopkins University Applied Physics Laboratory, put it this way: "It has given us quite a ride."

Comet ISON made a grand first impression.

When it was discovered in September 2012, it was unusually bright for an object so far from the sun. That led some observers to declare it the "comet of the century."

It might have lived up to that billing if it had continued to brighten as it made its way into the inner solar system. But that didn't happen. Scientists believe its initial brightness was probably due to a burst of gas that converted directly from a frozen-solid state after the comet encountered the sun's warmth for the first time.

When ISON passed by Mars, instruments aboard the Mars Reconnaissance Orbiter measured its nucleus as about two-thirds of a mile across, relatively small by comet standards. The nucleus is made of ice and space dust as fine as the dust you might find at the bottom of a vacuum cleaner, and it's about as dense as a well-packed snowbank.

"Picture it like the dirtiest snowball you've ever seen," Lisse said.

The snowball is surrounded by a gas bubble about 250,000 miles wide, roughly the distance from Earth to the moon. In recent images that bubble is glowing an emerald green, a result of the sun's radiation hitting the carbon gas released from the comet. The comet's tail is white and stretches a few million miles.

But there's more to Comet ISON _ named for the International Scientific Optical Network, a night-sky survey program managed by the Russian Academy of Sciences _ than good looks and pop culture appeal. It's a time capsule from the days before the planets.

Its orbit indicates it came from the Oort cloud, a giant sphere of icy objects that lies at the very edge of the solar system. Out there, the sun's gravity barely holds sway. When a star passes by the cloud, which happens every few million or billion years, the comets are perturbed and some get bumped into different orbits.

Comets in the Oort cloud formed billions of years ago at the dawn of the solar system. Some of their cousins joined forces to become the gaseous planets Jupiter, Saturn and Neptune; others were expelled into interstellar space. The rest, including ISON, have remained frozen and unchanged for milleniums.

"Think of this comet as a relic, a dinosaur bone of the solar system," Lisse said. "It has been in deep freeze for 4.5 billion years, and now we are watching it bake and boil as it moves closer to the sun."

Over the last year, scientists have kept a close eye on ISON and its jets of gas. That its jets have released lots of carbon dioxide, diatomic carbon, tricarbon, ammonia, methane and nitrogen was not a surprise; these molecules are necessary for life as we know it, and scientists have long thought that they were brought to Earth by comets. But the fact that ISON has very little carbon monoxide was unexpected.

"We're learning that comets do vary," said Lisse. "They are not made in the same shop."

The closer the comet gets, the more scientists will learn about its chemical makeup.

"Even the rocky bits will boil," Lisse said. "It's like when you put a metal bead in a flame in science class and saw all the different colors. The comet is going into the flame of the sun, and the stuff in the comet is going to boil and glow, and we are going to try to figure out what it is made of."

At 1:45 p.m. Eastern time, Comet ISON will make its closest solar approach, known as perihelion, coming within 684,000 miles of the sun, assuming it is still intact.

After that, one of three things can happen. Solar radiation and intense tidal forces may cause ISON to evaporate, never to be seen again. It might break apart into small chunks that may or may not be visible from Earth. Or it could emerge from its solar encounter and shed gas and dust so furiously that viewers will be able to see it even during daylight.

Whatever the outcome, Comet ISON has already allowed scientists to observe solar phenomena that are not usually visible.

Using telescopes designed to look at the area immediately around the sun, scientists have noticed the comet's tail wiggling like a windsock on an airfield. That movement is caused by the comet's ionized tail interacting with the solar wind, the stream of charged particles that blow off the sun, said Battams, who works in the Naval Research Laboratory's Space Science Division.

While skywatchers are pulling for ISON's survival _ "Come on ISON, beat the sun!" one fan wrote on Twitter on Wednesday morning _ there is still much to be learned if the comet's journey ends Thursday. If it does evaporate, Battams said, some of its particles may cling briefly to the complex magnetic field in the sun's atmosphere, causing it to glow and illuminate its structure.

The comet has moved too close to the sun to be visible from Earth, so for the next few days followers will have to rely on NASA's instruments to observe ISON for them. Scientists will know if the comet survived its brush with the sun within six hours of its closest approach.

If it does, it should be visible from Earth in the night sky in early December. The comet would be nearest to us on Dec. 26, when it zooms within 40 million miles of our planet.

And if ISON doesn't make it, scientists say they won't be too disappointed.

"We now have arguably the single largest cometary data set in history that we have to begin sorting, analyzing and writing about," Battams wrote on his blog. "Throughout all of 2014 there will be dozens of new scientific papers released ... and numerous talks, presentations and workshops. Even if comet ISON's show is nearly over, for us scientists it honestly is just the beginning."